Atomic Layer Deposition (ALD), originally called Atomic Layer Epitaxy (ALE), is a thin film deposition process used for over 30 years. Recently, this technique has gained significant interest in the semiconductor, data storage, optical, biomedical and energy generation industries. The films made by this technique have exceptional characteristics, such as being essentially pinhole free and possessing excellent step coverage even in structures with a very high aspect ratio. The ALD technique is also well suited for precise tailoring of material compositions and very thin films (<1 nm). High repeatability and relatively low cost makes the method also attractive to produce demanding optical filters and artificial, tailored materials in general.
ALD-type technologies are self-limiting processes. However, this does not mean that the resulting deposited material is uniform over the whole coated object. The laws of nature provide natural self-limiting borderlines for reactions, but circumstances (like temperature, chemistry, topography) on the surface sometimes vary, or the partial pressures or flows of chemical compounds are uneven, all resulting in varying uniformity.
In other words, ALD-type procedures include various reactions and effects causing uneven deposition. These non-uniformities can be problematic, for instance:                Uneven materials thickness or another material property reduces the production capacity or can prevent coating of large objects.        The thickened portions of the layer can adversely affect step coverage.        
Industrial applications require high uniformity and it is important to find ALD methods to achieve uniform properties for the deposited film.
A number of design principles for ALD tools are discussed below.
1) Cross-flow design, where gas flow carrying precursors enters from one side and the pumping exit is on another side. In addition to single wafer platforms, this mode can be applied for batch tools and tools for coating three-dimensional parts. Precursors can be fed through flow diffusers, showerheads, rotating pipes or through stationary pipes. The common flow paths for the precursors can be minimized or optimized.2) Top-shower-head design, where precursors are supplied above the area to be coated. This mode has limitations in batch applications when objects to be coated are stacked.3) Plasma can be applied directly above the object to be coated or be supplied from upstream. Plasma can be on during precursor dosing, or before or after the dosing to modify the coated surface. The use of plasma is difficult with batch tools where the objects to be coated are stacked, because the plasma may not distribute well and evenly inside the batch.4) Diffusion mode. For example, semiconductor memories and large-area substrates may have deep trenches, and to expose and purge these trenches a long time is required. To reduce precursor consumption, it is economical to reduce or even stop pumping during precursor dosing.5) The deposition tool can have hot or cold walls. Due to film growth on all exposed surfaces, the long-term practical use of thermal ALD typically requires hot wall. In plasma ALD tools, it may be possible to use the cold wall principle.6) Typically, ALD tools operate at vacuum pressures below 10 hPa. However, pressures up to and above atmospheric pressure are possible.7) Moving substrate. Typically, the object to be coated rotates.
At the moment, the semiconductor industry often uses single wafer or single pallet platforms. Most of ALD research is done without using batch. A reasonable amount of work has been done to improve the throughput of this kind of production tools. However, ALD is naturally compatible with large-area coating, because the deposition occurs on the surfaces exposed to precursors. The use of a batch or mini-batch system is economically more competitive. The display industry (for example Planar Systems Inc. TFEL production) has been using batch mode for over 20 years. Not much public material exists on batch systems, because research articles and most patents describe phenomena on single wafer systems. In practice, the batch ALD process often leads to more non-uniformity than does the same precursor chemistry when single wafer deposition tools are used. The cross-flow mode provides a relatively easy way to design batch tools, where objects are inside the coating volume, or inside the reaction chamber, installed on pallets or the like.
Accordingly, it is important to find ALD methods to achieve uniform film properties in a cross-flow batch mode.
Metal halides are good ALD precursors due to their high resistance against thermal decomposition. Metal halides are usually very reactive towards water. Metal chlorides are more common ALD precursors than metal iodides due to their higher vapor pressure.
Metal chlorides are also a low-cost source for metal atoms. Unfortunately, many ALD processes using chlorides are well known for the decreasing thickness in the precursor flow direction.
Accordingly, it is desirable to develop chloride-based ALD processes for obtaining high film uniformity.
Many ALD processes using water suffer from the variation of the growth rate due to variation of the amount of dosed water, and sometimes due to the purge time after the water dose. This does not mean that those processes are not ALD-type processes, but the self-saturation takes time and industrial processes often use shorter pulses than required for full self-saturation. This growth rate dependency on the water dose causes problems, especially with large coated areas and in batch setups. For example, differences in the water vapor flow causes varying local partial pressures, resulting in varying exposure of the surface to precursors. There is little uncertainty concerning the thermodynamic properties of water, which can be measured with more or less standard laboratory equipment. However, a lot of unsolved problems remain in the details of the water molecule structure and its relation to the forces acting between molecules. Water is a very polar molecule. It is generally believed, that the properties of water are simultaneously determined both by short-range attractive forces (identified as H-bonding), and strong long-range electrostatic (primarily dipole-dipole) interactions. The properties of water may cause molecular water on the surface after each water dose to an ALD tool, even at high temperatures.
The effect of increased ALD film growth rate vs. increased water dose is a well-known effect. One possible explanation for the increased rate is the occurrence of molecular water on the surface.
Generally speaking, most chloride-based ALD processes using water show a deposition thickness gradient, with thickness decreasing towards the flow exhaust side. It is important to notice that in addition to thickness non-uniformity, which is relatively easy to measure, there are many other material property non-uniformities. As examples may be mentioned refractive index, chlorine content, density, crystalline structure, permittivity, conductivity and work function.
Accordingly, it is desirable to develop ALD processes where the non-uniformity due to water vapor dosing and purging can be reduced.
A paper from Elers et al., Film Uniformity in Atomic Layer Deposition, Chem. Vap. Deposition 2006, 12, 13-24, presents an overview of the film uniformity challenges the industry is facing in the development of ALD processes. In particular, chapter 3.2. “Gaseous By-Products of the Surface Reaction” explains well the state of the art relating to this invention.
Concerning halide processes, the article mentions two important issues.
1. “One (little studied) source is a downstream non-uniformity issue caused by upstream reaction by-products. It has been speculated that the by-products can block active sites by adsorbing onto the surface, or even participate in a reverse reaction.”
2. “Although there are no comprehensive results to show how the uniformity degrades on the substrate, there is a strong assumption that non-uniformity is mainly caused by the hydrogen chloride by-product.” The article describes three proposed theories for explaining the non-uniformity; however, no methods to reduce the non-uniformity of halide processes are suggested.
Concerning metal alkoxide/H2O processes, the article mentions important issues.
“As with the metal halide processes, the metal alkoxide processes suffer from a small thickness non-uniformity, i.e., decreasing film thickness in the flow direction. As with HCl in the metal halide process, ethanol was proposed as a cause of the non-uniformity in the metal alkoxide process. The influence of by-products can be studied by introducing ethanol vapor with, or just after, the water vapor pulse. This increases the absolute amount of by-product species over the substrate and therefore reduces the relative concentration gradient of by-product species in the flow direction. Some preliminary results from such experiments have shown that the uniformity profile can be improved and even reversed, such that the film is thinner at the front-end than at the back-end of the substrate. In addition, a lower growth rate is observed when ethanol is introduced. Based on these observations, it has been speculated that ethanol can react with —OH sites, releasing water. A lower growth rate would then result from the decreased number of the reactive —OH sites. As in the metal halide process, it can be proposed that released water reacts with the metal alkoxide in the gas phase or in the reaction chamber surface, forming a particle.”
In the cross-flow reactor, the concentration of by-products increases in the direction of flow. By adding by-product (in this case ethanol) to the incoming flow, it is possible to cause this [speculated] adsorption and active site blocking effect also on the incoming side of the cross-flow reactor. The by-product of the reaction between metal alkoxide and water is alcohol, and the article describes a method to improve non-uniformity by supplying by-product, ethanol, together or after the water dose.
Rocklein et al (Conference presentation: AVS-ALD, Seoul Korea, Jul. 26, 2006) have disclosed a method called Compensated Atomic Layer Deposition, using HCl to compensate for inherent non-uniformities of deposition tool or process, thereby improving across-wafer uniformity and step coverage at the expense of deposition rate. In this particular connection, the HCl has been defined as a surface-poisoning gas.
In a paper from Utriainen et al., Controlled electrical conductivity in SnO2 thin films by oxygen or hydrocarbon assisted atomic layer epitaxy, Journal of the electrochemical society, 146 (1) 189-193 (1988), a method is described involving an ALD process using SnCl4 and H2O. Without modifications, the process results in a typical decreasing thickness profile in the flow direction. If hexane, C6H14, was dosed after the water, it reversed the usual deposition profile. Depositions were done at a temperature of 500° C. In studies made by the present inventors, it was not possible to observe clearly positive effects of hexane at 300° C. using a TiCl4+H2O+hexane sequence.
According to Ritala, Leskelä et al., Growth of titanium dioxide thin films by atomic layer epitaxy, Thin Solid Films 225 (1993) 288-295, a TiO2 deposition thickness profile can be avoided by leading the reactants to the reaction chamber through a common line. This is probably true as long as the effects from upstream (i.e. HCl gas, which is known to have an effect on layer uniformity) are large enough in relation to the required deposition area. In other words, whatever the upstream effect is, once its effect is large enough, it causes result similar to that of a long deposition length. If the deposition area is small related to the upstream area, then the measured film uniformity is better. However, up-scaled processes for large areas and batches have a clear uniformity profile and may require a sacrificial zone in front of the deposition area. This reduces capacity and increases cost.
The article explains growth rate variation when different glass substrates are arranged facing each other. The rate was different on different substrate materials, and especially interesting was that substrates were affected by the facing surface. The distance between substrates was 2 mm. However, the article does not teach how to decrease this non-uniformity.
The literature provides different opinions concerning molecular water on surfaces. According to Ritala, Academic Dissertation “Atomic Layer Epitaxy growth of Titanium, Zirconium and Hafnium dioxide thin films”. Helsinki, Finland 1994. ISBN 951-41-0755-1 page 35, “contribution of molecular water to the film growth can be considered insignificant at the temperatures where the saturated film growths took place”. This document also includes speculation regarding the reasons why the substrate affects film growth: “A more complicated question is how the substrate can affect the growth rate even long after it has become fully covered by the film. It seems that the only way to explain this is that the growth proceeds by a chain mechanism where the density of reactive sites left on the film surface after a deposition cycle is strictly related to their density prior to that cycle. Apparently, the existence of such a chain mechanism is more feasible if surface hydroxyls do act as intermediate species.”
However, the article does not teach how to decrease this non-uniformity.
Test runs carried out over the years by the present inventors have confirmed that the effects described by Ritala exist, and that they cause difficulties when surfaces are near each other. One example is shown in FIG. 1.
According to Kim, Property Improvement of Aluminium-Oxide Thin Films Deposited under Photon Radiation by Using Atomic Layer Deposition, Journal of the Korean Physical Society, Vol. 49, No. 3, September 2006, pp. 1271-1275, “For UV exposure after H2O injection (UV2), the hydroxyl groups weakly bonded to the surface may be removed by photolysis before the injection of TMA. Thus, film formation through the reaction between TMA and weakly bonded hydroxyl groups may be eliminated.” Kim used ultraviolet (UV) radiation to improve aluminium-oxide thin film properties in a trimethylaluminium-and-water ALD process. An important target of the work of Kim et al was to improve the properties of ALD aluminium oxide film by using UV radiation.
Several patents and applications mention the combined use of water and other oxidizers. For example in US20060035405 of Park, it is disclosed, that “oxidant may include ozone (O3), water (H2O) vapor, hydrogen peroxide (H2O2), methanol (CH3OH), ethanol (C2H5OH) and the like. These oxidants can be used alone, i.e., individually, or in combination with other suitable oxidants.” According to this document, the purpose of using several oxidizers is to improve oxidation. However, this prior art application does not provide any information on the possible benefits or effects when said combinations are carried out.
Application US 2006/0205227 from Sarigiannis describes a method where the normal ALD cycle using two precursors leaves residues, and a third gas is introduced to remove those residues. The application suggests the gases Cl2, O2 and H2 for removing residues. It is not explained what consequences such a combined use might cause, and it is not defined which combinations are preferred.
In U.S. Pat. No. 6,887,795, a process is disclosed for producing conductive thin films by producing a metal oxide layer by an ALD type process and essentially converting the metal oxide into an elemental metal by reduction using one or more organic compounds such as alcohols, aldehydes and carboxylic acids.
In US patent application publication 2007/0123060, a method is disclosed for enhancing the volatility of reactants and/or by-products in deposition processes by means of coordinating ligands, which are added to the sequence. The result is improved layer uniformity.
Various methods are used to affect surface water in technologies not directly related to ALD. For example, an interesting idea is to use a chemical having a particular affinity for water to react with the water adsorbed on a surface in such a way that the reaction products are gases, which may be pumped away. One such chemical is dichloropropane, and the reaction is (CH3)2CCl2+H2O=>(CH3)2C═O+2HCl. This technique is claimed (Tatenuma et al, J Vac Sci Technol A16, 1998, 263) to reduce the base pressure in a vacuum system by factor of 80. This or similar methods or chemicals may be usable with ALD technology for changing film properties.
In batch setups, several phenomena occur which are not easily visible from the results of single wafer tool processes.                Small distances between surfaces may cause strange effects. In some cases, for example with TiCl4+H2O processes, the growing film is affected by the opposite surface and some of its properties. Features of the opposite side are visible in the deposited film structure. The visible effect can be for example the change of the refractive index and/or film thickness. This effect causes difficulties to increase deposition capacity. The reason for this effect is unknown. It may be that the effect is caused by electrical fields from functional OH groups and/or polar water molecules attached on the surfaces. The effect of the deposition surface, under the growing film, on growth rate is well known to ALD workers.        Decreased flow channel cross section area between surfaces increase by-product partial vapor pressure. Thus all effects from by-products are the stronger, the smaller is the distance (and the higher the deposition capacity) between surfaces.        An enlarged deposition area increases by-product partial vapor pressures and causes effects from by-products, similar to those of a decreased flow channel area.        Non-uniformity eventually causes flaking of the cumulative film on the coated jigs, walls and other construction parts. Flaking typically starts from the inlet side, where films are thicker. This causes increased cost due to required cleaning.        
Alcohols are widely used as solvents. Generally, hydroxyl group compounds are polar, which tends to promote solubility in water. But the carbon chain resists solubility in water. Short chain alcohols (methanol, ethanol, and propanol), in which the hydroxyl group predominates, are miscible in water. Butanol is moderately soluble because of the balance between the two opposing solubility trends. Higher alcohols are practically insoluble in water because the influence of the hydrocarbon chain is stronger.
The existing literature gives several opinions and facts concerning non-uniformity in cross flow halide—water processes as described above.
It has been speculated that the by-products can block active sites by adsorbing onto the surface, or even participate in a reverse reaction. In the cross-flow reactor, the concentration of by-products increases in the direction of flow. For example in the reaction TiCl4+H2O=>TiO2+HCl it is speculated that the HCl or chlorine attaches to the surface, reserving surface sites and thus causing decreasing thickness in the flow direction. Similarly, ethanol is proposed as a cause of the non-uniformity in the metal alkoxide processes.
It is also possible that molecular water is attached to a surface. Water is a very polar molecule. With deposition using chlorides it may be that the reaction by-product HCl removes “loose molecular water” from the surface. HCl is a polar molecule.
A known method to improve uniformity is to use HCl gas. A dose of HCl after a H2O or halide dose might increase the effects of HCl also on the upstream zone, which otherwise would be stronger on the downstream zone. However, HCl is a dangerous, corrosive gas and thus often not accepted in the production environment.